Graft copolymers of polypropylene have been of interest for some time because they are capable of possessing some properties of the grafted polymer, produced by the homo or copolymerization of the monomer or monomers, respectively, as well as of the polypropylene backbone. Some of these graft copolymers are useful as compatibilizers in the preparation of normally incompatible polymer blends or alloys.
Polypropylene graft copolymers generally have been made by forming active sites on polypropylene by irradiation with high-energy ionizing radiation either in the presence of a monomer capable of grafting to the active sites, or followed by treatment with the monomer. The free radicals produced in the propylene polymer as a result of the irradiation act as initiators for the polymerization of the monomer, as well as active sites for grafting.
Much of the prior art related to the radiation grafting of polymerizable monomers onto polypropylene describes applying the process to already shaped, i.e., prefabricated, polypropylene articles, such as films, filaments, woven fabrics and the like. While occasional reference has been made heretofore to the use of polypropylene in powder or granulate form as the substrate for the graft copolymerization, attention has not been centered on possible effects that the grafting process conditions may have on those properties of the resulting graft copolymer which are important in determining its processibility. For example, some of the conditions disclosed in the prior art lead to drastic reductions in the length of the polypropylene backbone, while others lead to crosslinking. Such occurrences often are unacceptable, for example when the graft copolymer is in the form of particulate material which has to undergo melt-processing to be formed into articles of various types.
U.S. Pat. No. 3,058,950relating to the preparation of dye-receptive graft copolymers of N-vinyl-3-morpholinone on polyolefin substrates, by the use of high-energy radiation, states that it is generally desirable, and of significant advantage, to employ the polyolefin as a pre-formed, fabricated article. This reference also states that unfabricated graft copolymers made by the described process may be converted to shaped articles by any desired technique adapted for such purpose with conventional polymers. Grafting process features described as advantageous include diluting the monomer in a solvent or dispersant, immersing the polyolefin polymer substrate in the monomer solution or dispersion bath, and subjecting the monomer-impregnated polyolefin substrate to a field of high-energy radiation. These teachings are indicative of a failure to recognize that one or more of the conditions disclosed as being utilizable, and even advantageous, in the graft copolymerization process may have a deleterious effect on the processibility and/or utility of the graft copolymer.
According to U.S. Pat. No. 3,714,083, polypropylene powder is irradiated in air at a temperature below 5.degree. C., preferably within the range of -20.degree. to -40.degree. C., and then placed in a dilute solution of divinylbenzene monomer in a solvent such as methanol. Higher irradiation temperatures are said to cause a predominance of homopolymer formation, and irradiation at above 5 Mrad is stated to be undesirable because of excessive breakage and degradation of polypropylene that is said to occur even at temperatures as low as 5.degree. C. or less.
The effect of oxygen in one stage or another of the radiation initiated grafting process is variously described in the prior art. For example, in the process described in U.S. Pat. No. 3,201,336, polypropylene, preferably in the form of a semi-finished or finished article, is subjected to high-energy ionizing radiation in the presence of oxygen, and thereafter the irradiated polymer is contacted with different monomers in successive stages to produce different graft branches on the polypropylene trunk. The presence of oxygen is said to be required to form active centers at which the grafting will take place. Oxygen, said to be a polymerization inhibitor, preferably is absent during the contacting of the monomer with the polymer.
In U.S. Pat. No. 3,188,165, the use of an atmosphere of inert gas, or an air- and water-impervious wrap, around a monomer-treated shaped substrate during irradiation to avoid the degradation which may occasionally be observed when the irradiation is conducted in the presence of air or moisture is disclosed.
U.S. Pat. No. 3,314,904 describes blending (a) a graft copolymer of styrene, or styrene and acrylonitrile, on polyethylene or polypropylene and (b) a compatible rubber to produce a "gum plastic". The graft copolymer is made by first "activating" the polyolefin by subjecting it to high-energy ionizing radiation, and then contacting the irradiated polymer with the monomer and subjecting the mixture to polymerizing conditions. Linear polypropylene of large surface area per unit weight is recommended as a substrate. Pro-fax 6501 propylene homopolymer is specified as typical. To maintain graftability, the irradiated polypropylene is kept cold and in an inert atmosphere until it is charged into the grafting reactor. To maximize the styrene or styrene/acrylonitrile content (75-95% by weight) in the graft copolymer, the irradiated polypropylene is stirred with a dilute solution of the monomer(s) at elevated temperatures for long periods of time. The monomer conversion, i.e., the amount of monomer consumed to form the graft copolymer, is low.
East German Patent 135,499 describes a radiation grafting process. It involves bringing one or more normally liquid monomers in the vapor phase, on a carrier gas, into contact with a polyolefin powder or granulate, and subsequently removing unreacted monomer from the reaction zone with the carrier gas. The monomer vapor can be introduced after completion of the irradiation or before the irradiation commences, or the irradiation and introduction of monomer vapor can begin together. Also, additional monomer vapor can be introduced after completion of the irradiation. The product made from polypropylene and styrene and acrylonitrile vapors by this process was a mixture of polypropylene and a graft copolymer of propylene and styrene/acrylonitrile, the grafting having taken place on the external surface of the polypropylene granulate.
A process is needed for performing the radiation grafting of polymerizable monomers onto olefin polymers in raw, as-polymerized, or unprocessed form, i.e., onto virgin polymer, which minimizes degradation or visbreaking and does not lead to crosslinking of the polymer substrate. Degradation has a deleterious effect on the molecular weight of the polymer, and crosslinking adversely affects or destroys the melt processability of the polymer. In other words, the process should produce a polyolefin graft copolymer wherein the molecular weight of the olefin polymer backbone and melt processibility of the graft copolymer are comparable to those of the olefin polymer starting material used to form it. In addition, the process should not result in a graft copolymer having a melt flow rate which increases on storage due to the presence of residual free radicals. Further, a graft polymerization process is needed which is relatively easy to practice, which utilizes the grafting monomer in an efficient manner so that the formation of homopolymer of the grafting monomer at the expense of the formation of the poly(monomer) grafted olefin copolymer is minimized, and which is more economical than current processes.
There is also a need for graft copolymers of polyolefins in the form of particles having a uniform distribution of graft polymerized monomer throughout the particle. Graft copolymers having uniform distribution are advantageous in that they afford grafted olefin polymer products whose properties are unaffected by the presence of relatively large volumes of essentially unreacted olefin polymer in the grafted olefin polymer particles, which happens in conventional graft polymerization processes since the monomer essentially grafts to the surface of the polymer particle thereby producing a shell of grafted polymer around an essentially ungrafted olefin polymer core.